4,4′-BS is an important compound in the fields of chemical industry involving textiles, resins, etc. Recently, 4,4′-BS of higher purity is required in these fields.
A method that has been employed is a “method for isolating 4,4′-BS from a dihydroxydiphenylsulfone isomeric mixture comprising treating a dihydroxydiphenylsulfone isomeric mixture with a mixed solvent composed of 80-40 wt. % of o-dichlorobenzene and 20-60 wt. % of phenol, and filtering off the resulting product without heating to isolate 4,4′-BS” (Japanese Patent Publication No. 1976-36264).
However this method does not give 4,4′-BS in high yields.
Another method that has been employed is a “method for synthesizing 4,4′-BS comprising reacting phenol with sulfuric acid in an inert solvent having a weight about 15 to about 35% of the weight of the reactants to produce an isomeric mixture, removing water from the reaction system at about 160 to about 200° C. by heating, maintaining the temperature of the resulting reaction mixture at about 160 to about 200° C. until the generation of water terminates, cooling the water-removed reaction mixture to about 80 to about 120° C., adding if necessary a replenishing amount of the inert solvent, forming a slurry in which the weight of the inert solvent accounts for about 20 to about 80% of the reaction mixture, and separating the slurry into crystalline 4,4′-BS and a liquid in which 2,4′-BS is dissolved (see Japanese Patent Publication No. 1992-74347).
According to Japanese Patent Publication No. 1992-74347, 4,4′-BS can be obtained with purities of 93.5 wt. % (Example 1) and 95.3 wt. % (Example 2). Japanese Patent Publication No. 1992-74347 does not disclose a method that can produce 4,4′-BS of higher purity. Moreover, although the examples in Patent Publication 2 recite that “by-products were not detected by gas chromatography”, phenolsulfonic acid undetectable by gas chromatography often remains as an impurity.